JBDS is by nature more stable than JBoss Tools, which incubates new code and projects until they can be released into JBDS. (Think Fedora vs. RHEL.)
1) you can't safely upgrade from JBDS 2.1 to JBoss Tools 3.1 because they depend on different versions of Eclipse (3.4.2 vs. 3.5.x). You can, however, reuse an existing workspace and all your existing projects - but you need a fresh Eclipse + JBoss Tools install.
If you want to live on the edge, you can use Eclipse 3.5.1 with JBoss Tools 3.1.0.M4 (or even a newer nightly build if you want the truly bleeding edge).
2) JBDS 3.0 will support Maven2 through the m2eclipse project; that work is already under way in JBoss Tools 3.1.0.M4.
3) Much of this is coming soon, but I can't say specifically what or when just yet. Others may have a better idea.
3) There are a ton and a half of awesome things coming down the pipeline. JSF 2 (with its myriad of things). Bean Validation. CDI w/Weld. RichFaces 4. Seam 3. The first three, of course, having RIs already released! Is there tooling for those that are already released? If so, is there documentation or presentations on how to use these tooling features?
We support Composition Components in VPE (visual templates and code completion) in JBT 3.1 M4. See https://jira.jboss.org/jira/browse/JBIDE-4970
And we are going to continue working on JSF 2 tooling. See http://www.jboss.org/community/wiki/JSF2supportinJBT
Also we are working on CDI/Weld tooling right now. See http://www.jboss.org/community/wiki/contextsanddependencyinjectionjsr-299seam3toolsfeatures
And of course will add new features for RichFaces 4 and Seam 3 too.